U.S. patent number 7,988,937 [Application Number 12/807,275] was granted by the patent office on 2011-08-02 for decontamination of radioactive metals.
Invention is credited to David S. Eaker, Rick Low, W. Novis Smith.
United States Patent |
7,988,937 |
Smith , et al. |
August 2, 2011 |
Decontamination of radioactive metals
Abstract
The present invention relates to a method for the volumetric
decontamination of radioactive metals. The method includes the step
of precipitating out radioactive gamma and beta emitting
nucleotides and then recovering non-radioactive metal
compounds.
Inventors: |
Smith; W. Novis (Philadelphia,
PA), Eaker; David S. (Lenoir City, TN), Low; Rick
(Lenoir City, TN) |
Family
ID: |
44314331 |
Appl.
No.: |
12/807,275 |
Filed: |
September 1, 2010 |
Current U.S.
Class: |
423/2; 423/11;
423/7; 423/6; 423/18 |
Current CPC
Class: |
C01G
21/003 (20130101); G21F 9/12 (20130101); C01G
11/003 (20130101); G21F 9/125 (20130101); C01G
45/003 (20130101); C01G 9/003 (20130101); C01G
49/0009 (20130101); G21C 19/46 (20130101); C01G
5/003 (20130101); C01G 3/003 (20130101); C01G
53/003 (20130101); Y02E 30/30 (20130101); Y02W
30/50 (20150501) |
Current International
Class: |
C01G
56/00 (20060101) |
Field of
Search: |
;423/2,6,7,11,18 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1211816 |
|
Nov 1970 |
|
GB |
|
62-297219 |
|
Dec 1987 |
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JP |
|
Primary Examiner: Bos; Steven
Attorney, Agent or Firm: Lezdey; John
Claims
What is claimed is:
1. A method for the decontamination of radioactive metals which
comprises the steps of: A) dissolving said radioactive metals in an
aqueous acid solution which provides an oxidative environment in
the presence of about 0.1 to 1% by weight of iron particles; B)
neutralizing the solution from step A) to a pH of about 3 to 4 to
form a gelatinous precipitate which contains radioactive gamma
emitting nucleotides and beta emitting nucleotides and an iron
compound; C) filtering the product of step B) and passing the
filtrate through a strong anion exchange column; D) neutralizing
the filtrate from step C) to a pH of about 10-12, optionally
concentrating the filtrate from step C); and then E) recovering any
metal compounds from step D).
2. The method of claim 1 wherein the radioactive metal is selected
from the group consisting of nickel, lead, cadmium, zinc, copper,
manganese and silver.
3. The method of claim 2 wherein the radioactive metal is
nickel.
4. The method of claim 1 wherein the anion exchange column is
conditioned with a member selected from sodium nitrate or sodium
sulfate solution at a pH of about 3-4.
5. The method of claim 1 wherein the acid solution of step A)
comprises nitric acid and sulfuric acid.
6. The method of claim 1 wherein the filtrate from step C) is
passed through a strong anionic exchange column which has been
conditioned with a solution having a pH of about 3-4 and then
proceeding to step D).
7. The method of claim 1 wherein the acid solution of step A)
comprises an acid selected from the group consisting hydrochloric
acid, acetic acid, formic acid, citric acid and lower molecular
weight carboxylic acid having up to six carbon atoms.
8. The method of claim 1 wherein the radioactive metal is nickel
and sodium hydroxide solution is added in step D) to precipitate
out nickel hydroxide.
9. The method of claim 1 wherein the radioactive metal is nickel
and nitric acid is used to dissolve the nickel metal in step
A).
10. The method of claim 1 wherein the anionic exchange column
comprises a chloride anionic resin.
11. The method of claim 1 wherein the metal compound of step E) is
converted to a metal state.
12. A method for the decontamination of radioactive nickel which
comprises the steps of; a) dissolving said radioactive nickel in an
aqueous acid solution selected from the group consisting of
sulfuric acid and nitric acid in the presence of about 0.1 to 1% by
weight of iron particles; b) neutralizing the solution from step a)
with a base to a pH of about 3 to 4 to form a gelatinous
precipitate containing gamma and beta emitting nucleotides and an
iron compound; c) filtering the product of step b) and passing the
filtrate through a strong anionic exchange column which has been
conditioned to a pH of about 4 with a solution of sodium sulfate or
sodium nitrate; d) optionally concentrating the filtrate of step c)
to obtain nickel sulfate or nickel nitrate, or neutralizing the
filtrate of step c) with a base to obtain a nickel compound
precipitate, and then e) recovering the nickel compound.
13. The method of claim 12 wherein step d) comprises neutralizing
the filtrate from step c) to a pH of about 10 to 12 to obtain a
nickel compound.
14. The method of claim 13 wherein the nickel compound is nickel
hydroxide.
15. The method of claim 13 wherein the filtrate is neutralized with
an alkali compound selected from the group consisting of hydroxide,
carbonate, sulfide, stearate, oleate and oxalate.
16. The method of claim 12 wherein technetium 99 is removed.
Description
FIELD OF THE INVENTION
The present invention relates to the decontamination and conversion
of volumetrically radioactive metals, especially nickel and its
alloys. According to the invention the metals in the form of
filings, powders, shots, flakes or the like are treated in an
aqueous acid solution or additive environment. The resulting
product is then either passed through a conditioned anionic
exchange column to provide a decontaminated salt solution or
neutralized to a pH of about 3-4 to precipitate out the radioactive
nucleotides and then passing through the anion exchange column.
BACKGROUND OF THE INVENTION
High purity metals and alloys are used in the upgrading and
processing of nuclear fuels and the containment of nuclear reactor
fuel rods. These same metals and their alloys are used in the
containment of the actual reactors both for power generation and
for military vessels such as submarines. These same metals during
the course of their service come under constant high radiation
doses and accumulate low to moderate levels of radioactivity within
the metals itself. This service acquired inherent radioactivity is
termed "volumetric radioactivity" as opposed to surface
radioactivity which is due to radioactive particles on the surface
of the metal form or part. The surface radioactivity can be washed
or scoured off while most of the volumetric radioactivity cannot be
removed by physical processes such as washing or heating even up to
the vary high melting point of the metal or metal alloy. Nickel and
its alloys, for example, are widely used for these applications and
accumulate various levels of volumetric radiation due to technetium
99, a beta radiation emitter and other radioactive isotopes. These
radioactive isotopes include both gamma and beta emitters which are
of concern along with the weaker alpha emitters. The removal of the
volumetric radioactive isotopes from the volumetrically radioactive
contaminated nickel and its alloys is of significant importance in
order to recover this valuable metal. Since nickel is the preferred
containment material for nuclear processing and reactors,
volumetrically radioactive contaminated nickel will continue to be
generated. The reuse of this contaminated nickel by re-melting it
and restricting it only for nuclear use can be done but there is a
quantity mismatch since there is a very large inventory of this
volumetrically contaminated nickel already being stockpiled
awaiting some means of purifying it back to background levels and
for reuse in commercial applications. This nickel at its current
volumetric radioactive levels cannot be recycled back into the
normal commercial metal alloying and fabrication processes. This
nickel and its alloys continues to be generated as older nuclear
reactors and older nuclear fuel processing facilities are
decommissioned. Not only is the economic loss and value of major
concern, the necessity to recycle and reuse the limited amount of
metals such as nickel is a worldwide concern in order to maintain
the long term availability of such important metals.
Various decontamination processes are known in the art, and
specifically for decontamination of nickel. Nickel can be removed
by selectively stripping from an acidic solution by electrowinning.
See U.S. Pat. No. 3,853,725. Nickel may also be removed by liquid,
liquid extraction or solvent extraction. See U.S. Pat. Nos.
4,162,296 and 4,196,076. Further, various phosphate type compounds
have been used in the removal of nickel. See U.S. Pat. Nos.
4,162,296; 4,624,703; 4,718,996; 4,528,165 and 4,808,034.
It is known that metallic nickel, contaminated with fission
products, could be decontaminated to remove any actinides present
by direct electro-refining based on the differences in reduction
potential in the electromotive force (emf) series. Actinide removal
is favored by two phenomena during electro-refining. Actinides have
a significantly higher reduction potential relative to nickel and
they are normally won from molten salt electrolyte rather than from
aqueous electrolyte. See U.S. Pat. Nos. 3,928,153 and 3,891,741,
for example.
In spite of these disclosures, there remains a need for an
economical and efficient method to decontaminate metals.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method for
the decontamination of radioactive metals. The method comprises the
steps of: A) dissolving the radioactive metals in an aqueous acid
solution which provides an oxidative atmosphere in the presence of
about 0.1 to 1% of iron particles; B) neutralizing the solution
from step A) to a pH of about 3 to 4 to form a gelatinous
precipitate which contains radioactive gamma emitting nucleotides
and beta emitting nucleotides and iron hydroxide; C) filtering the
product of step B) and optionally passing the filtrate through a
strong anion exchange column which has been conditioned with a
solution having a pH of about 3-4; D) neutralizing the filtrate
from step C) to a pH of about 10-12 to form a hydroxide or other
salt and then recovering the metal hydroxide or metal salt from the
filtrate of step C).
Advantageously, the anion exchange column comprises a sulfate anion
resin.
It is an object of the invention to decontaminate radioactive
metals.
It is another object of the invention to decontaminate radioactive
nickel and form nickel hydroxide.
It is a further object of the invention to maintain any technetium
present in the pertechnate anion state for absorption on an anion
exchange column.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to a method for the decontamination
of radioactive metals and to form metal compounds which, if
desired, can be converted to non-radioactive metals.
According to the method the radioactive metal in the form of
filings, powder, shot or the like is subject to the following
steps: A. Dissolve the radioactive metals in an aqueous acid
solution which provides an oxidative environment in the presence of
about 0.1 to 1% by weight of iron particles. In some cases nickel
metal already has the iron present. B. Neutralizing the solution
from step A) with a base to a pH of about 3 to 4 to form a
gelatinous precipitate which contains radioactive gamma emitting
nucleotides and beta emitting nucleotides and an iron compound.
Potassium hydroxide and sodium hydroxide are the preferred bases.
C. Filtering the product of step B) or optionally passing the
filtrate through a strong anionic exchange column which has been
conditioned with a solution having a pH of about 4 with a sulfate
or nitrate solution. The anionic exchange resin is preferably
SIR-110-HP of ResinTech of New Jersey which initially is a chloride
anionic resin. Other strong anionic exchange resins can be used
such as DOW 550A from Dow Chemical Company. The anionic exchange
column is preferably conditioned prior to use with a concentrated
solution of sodium sulfate or sodium nitrate at a pH of about 3 to
4. (The metal sulfate or nitrate salt can be recovered at this
point by concentration of the filtrate or optionally); D.
Neutralizing the filtrate to a pH of about 10-12 to form a
hydroxide compound. Preferably a base such as sodium or potassium
hydroxide is utilized.
Preferably, the radioactive metal is dissolved in an acid solution
of about 0.5 to 2.5M. In the case of nickel, the upper
concentration is determined and limited by the solubility of the
nickel salt. The temperature can be maintained from about
20.degree. C. to 100.degree. C. depending on the rate of addition
and the rate of reaction desired.
There is a need to maintain an oxidative environment to ensure that
the technetium is maintained in the oxidized state or in the
pertechnate anion state for absorption on the anionic exchange
column if used for removal. The presence of nitric acid maintains
this oxidative environment even up to pH 4 for column absorption.
Since hydrochloric acid and acetic acid provide a reducing
environment in the strong acid, these will not ensure that the
technetium cation or anion is maintained or even converted to the
desired pertechnate anion for removal. However, the other acids
such a hydrochloric acid, acetic acid, formic acid, citric acid,
and lower molecular weight carboxylic acid having up to six carbon
atoms can be used with other metals such as lead which do not have
isotopes of technetium. It is important that about 0.1 to 1% of
iron particles are added to the initial slurry of dissolving metals
to provide a small amount of soluble iron salts, preferably the
nitrate or sulfate. If the nickel or other metals contain a small
amount of iron, additional iron is not required.
After the metals have been dissolved in the acid solution, the
solution is neutralized with a hydroxide base to a pH of about 3 to
4 to form a gelatinous precipitate of ferric iron and any aluminum
hydroxide. This gelatinous precipitate occludes, absorbs, and drags
down substantially all of the radioactive gamma and beta emitting
nucleotides including technetium 99. The resulting slurry of
gelatinous precipitate is filtered to produce a solution of the
metal salts or compounds. The small amount of gelatinous
precipitate has a high level of radioactivity and must be handled
with appropriate procedures depending on the level of radioactivity
of the starting metal. The gelatinous material can be readily
disposed of or used to recover any isotopes.
The filtrate can be either concentrated to recover the metal salts
or passed through a conditioned ion exchange column to further
remove technetium 99. The ion exchange column can be any anion from
hydroxide to a chloride. A hydroxide column is basic and needs to
be changed to the sulfate to nitrate anion at a pH of about 3 to 4
so as not to precipitate nickel hydroxide on the column which
starts to precipitate at a pH of about 4.5. The chloride anion must
be removed since it acts as a reducing forming chlorine in presence
of oxidized forms of metal cations such as the pertechnate. The
chloride anion would destroy this necessary oxidized anion which is
readily absorbed on the ion exchange column at the pH 3-4.5
conditions. Therefore the column must be preconditioned with a
concentrated sodium of pH 3-4 sodium sulfate or nitrate. After the
filtrate of sodium salt has been processed down through the anion
exchange column, the solution is essentially free of technetium 99.
However the top of the anion exchange column shows presence of the
technetium 99.
The filtrate from the ion exchange column can be concentrated to
recover the metal salts or be treated with a alkali hydroxide to
form hydroxide compounds, especially to form nickel hydroxide which
can be used in batteries.
The metal forms can be obtained by electrowinning as described in
U.S. Pat. No. 5,156,722, which is herein incorporated by reference
or electro-refining the decontaminated metal compounds described in
U.S. Pat. No. 5,183,541 which is incorporated herein by
reference.
Example 1
An anion exchange column which was 9'' long and 1 inch in diameter
was filled with 65 g of Dow 550A which is an anion exchange resin.
The resin was conditioned by passing a concentrated solution of
sodium sulfate at a pH of 4, three times through the column at room
temperature.
Separately, 100 g of radioactive nickel filings and 1 g of iron
filings were slurred in 500 ml of water. A mixture of sulfuric acid
and nitric acid was slowly added to the slurry with stirring until
all of the metal was dissolved.
The resultant solution was neutralized to a pH of about 4.5 with
sodium hydroxide solution to form a gelatinous precipitate which
contained the radioactive nucleotides. This solution was then
filtered. The filtrate was passed through a sulfate preconditioned
(pH 4) anion exchange column (SIR-110-48 of ResinTech, NS).
The filtrate was then neutralized to pH 10-11 to precipitate out
nickel hydroxide. This slurry was filtered and the nickel hydroxide
recovered and dried. There was no detectable gamma or beta
radiation.
Alternatively, sodium carbonate can be used to precipitate nickel
carbonate.
The gamma and beta emissions were measured in disintegrations per
minute (dpm).
Results
TABLE-US-00001 TABLE I Beta and Gamma Levels Achieved Beta Levels
(pCi/g) Gamma (minus Ci/ml) 1. Starting Nickel Tc 99 60 3 .times.
10.sup.-4 metal filings 2. Ni (NO.sub.3).sub.2 isolated -- -- 1
.times. 10.sup.-6 from filtrate from (background) iron hydroxide
precipitation (pH 4.5) 3. Iron Hydroxide -- -- 1.5 .times.
10.sup.-5 Gel (higher for small material) 4. Product Ni (OH).sub.2
1.9 1.9 background level 5. White salt from non 7.1 background
level neutralized process detectable filtrate after Nickel
hydroxide removal 6. Top of ion 54 10 -- exchange column 7. Bottom
of ion 2.9 4.7 -- exchange column *General acceptable level for
Beta Is less than 26 pCi/g - Process accomplished this. Note: Alpha
was non detectable in Ni (OH).sub.2 The readings were based on a
Geiger counter.
Example 2
To a reaction beaker containing 100 g of nickel filings having a
radioactivity of the gamma nucleotides of 3 micro curies/ml and of
the beta nucleotide 0.04 micro curies/ml was added 70% nitric acid
which was made up with 800 ml of water in three portions. After 8
minutes from the addition of the first portion, the exothermic
reaction started. After another 15 minutes, the second portion was
added. The third portion was added after another 15 minutes. The
reaction was maintained at a temperature of 60-70.degree. C. After
about 4 hours, the solution was neutralized to a pH of 4.5 and left
overnight to cool. A gelatinous precipitate formed which was
filtered off and had a gamma nucleotide count of 1.46 micro curies
and a beta nucleotide count of 0.05 micro curies. The solution
which contained a nickel compound had no detectable gamma count.
The solution was added to a beaker and 40% sodium hydroxide was
added and the solution was brought up to a pH of 11.5 in which
nickel hydroxide precipitated out. The precipitate was dried and
tested. The beta and gamma were at background levels. There was no
beta or gamma emissions in either the filter cake. The filtrate
containing sodium nitrate was neutralized and showed background for
beta emissions.
It is to be understood that the above-described arrangements are
only illustrative of the application of the principles of the
present invention. Numerous modifications and alternative
arrangements may be devised by those skilled in the art without
departing from the spirit and scope of the present invention and
the appended claims are intended to cover such modifications and
arrangements. Thus, while the present invention has been fully
described above with particularity and detail in connection with
what is presently deemed to be the most practical and useful
embodiments of the invention, it sill be apparent to those of
ordinary skill in the art that numerous modifications, including,
but not limited to, variations in materials, form, function and
manner of operation and use may be made, without departing from the
principles and concepts of the invention as set forth in the
claims.
* * * * *